Production of lower vinyl esters of carboxylic acid e.g. vinyl acetate from ethylene and acetic acid has been known for a long time. This reaction e.g. to vinyl acetate monomer (VAM), is carried out, usually, in gas phase using noble metal catalysts. However, such processes are not realized easily on an industrial scale when involving higher carboxylic acids. Higher boiling points of these carboxylic acids make them difficult to gasify in scalable amounts and for the same reason it is difficult to separate the vinyl esters from carboxylic acids. For these reasons, vinyl esters of higher carboxylic acids are commonly produced either by trans vinylation involving vinyl acetate and higher carboxylic acid, or by Reppe vinylation involving acetylene and higher carboxylic acid. Major drawback of transvinylation is that it needs two consecutive reactions (i) vinylation or acetoxylation step, that is the production of vinyl acetate and (ii) trans vinylation, which generates stoichiometric amount of acetic acid as side product.
Most producers of vinyl esters of higher carboxylic acids use Zn-catalyzed Reppe vinylation because the process operates by the use of a cheap Zn catalyst. However, Reppe vinylation suffers with drawbacks with respect to the raw materials as well as operational issues. Main drawback of Reppe vinylation is the use of the unsustainable reactant, acetylene as raw material. Reppe vinylation to produce vinyl esters involves high concentrations of solid catalyst, in the ranges of 30 to 70 wt %. This high concentration of solid catalyst may lead to clogging and in turn creates operational issues for the process. Further the prior art on Reppe vinylation, U.S. Pat. No. 6,891,052 and U.S. Pat. No. 1,786,647, the Zinc-catalyzed processes are known to produce acid anhydrides of carboxylic acid as by-products. However, the degree of acid anhydrides formation varies depending upon the type of carboxylic acid and the reaction conditions such as temperature. In the same art, the methods are known to convert the acid anhydride back into the carboxylic acid with the addition of water during the vinylation, although this is not without affecting the catalyst activity.
To overcome the above problems, there have been few attempts to carry out the vinylation of higher carboxylic acids in liquid phase conditions involving ethylene as raw material. They are described in U.S. Pat. No. 3,221,045 and EP0648734. U.S. Pat. No. 3,221,045 is one of the first patents to describe the vinylation process using ethylene and carboxylic acid in the presence of Pd catalyst and Cu co-catalyst system under liquid phase reaction conditions. Air or oxygen was used as terminal oxidant for the Cu-catalyst. Results demonstrated the vinylation in the absence of air while oxidizing Pd catalyst off-line in the presence of Cu co-catalyst with air or oxygen. The reported vinylation of carboxylic acids in the absence of air is of importance with respect to the process operation. However, the catalyst system displayed low catalyst activity and, in turn, this process produced low yields of vinyl esters. EP 0648734 claimed an improved process to produce vinyl esters under liquid phase reaction conditions from ethylene and higher carboxylic acids. This process used Pd(0) colloid, stabilized by a polyvinyl pyrrolidone polymer in combination with Cu as a co-catalyst and LiCl and diglyme as promoters, with the terminal oxidant being either molecular oxygen or air. This vinylation process reported low yields in the absence of solvent, while above 50% yields in the presence of solvents. These solvents include diglyme, t-amyl alchol, t-butanol, tetrahydrofuran, and methyl propanol. The process claimed the use of Cu(II)-2-ethyl hexanoate as co-catalyst, and which resulted to increase the yields of vinyl esters. However due to the use of Cu(II)-2-ethyl hexanoate as co-catalyst the results also showed formation of considerable amounts of vinyl ester of 2-ethyl-hexanoacid as by-product along with desired vinyl ester. Additives LiCl and diglyme helped to increase the yield of vinyl ester.
In the hands of current inventors, the vinylation process with Pd(0) colloid stabilized by polyvinyl pyrrolidone polymer in the combination of Cu(II)-2-ethyl hexanoate co-catalyst was suffering from severe formation of Cu containing solid green deposits during and after reaction. Furthermore, the process was suffering from drawbacks such as (i) the use of (volatile) additives (co-solvents) during carboxylation, around 10 to 50 weight %, (ii) the difficult separation of the product from the reaction mixture because of contamination with the (volatile) additives and (iii) the limitations in concentration of Pd catalyst and Cu co-catalyst due to their limited solubility in the reaction liquid phase. In the absence of the mentioned polyvinyl pyrrolidone polymer, this process showed higher rate of vinyl ester formation, but also suffered from drawbacks such as (iv) poor stability of catalyst system, as reflected by formation of solid black-green deposits of Pd catalyst and/or Cu co-catalyst already before as well as during and after the reaction, resulting in loss of catalyst material and catalytic activity, (v) problems with the recovery of catalyst material and of catalyst activity after the reaction. In addition to the above, this patent demonstrated good yields of vinyl ester only with Cu(II)ethylhexanoate as co-catalyst precursor for different carboxylic acids. (vi) Use of Cu(II) 2-ethylhexanoate leads to concomitant formation of vinyl ester of 2-ethylhexanoic acid as by-product along with desired vinyl ester of carboxylic acid. This introduces another drawback (vii) because this mixture, containing the desired vinyl carboxylate and undesired vinyl 2-ethylhexanonate, results in a more complicated separation process of the desired vinyl ester. Further it is known art that (viii) the stabilizing agent such as polyvinyl pyrrolidone polymer is known to undergo degradation in the oxidation environments such as in vinylation reaction [Environ. Sci. Technol., 2014, 48 (22), page 13419-13426]. On a whole, the catalyst system containing Pd colloid stabilized by PVP in the combination of Cu(II)-2-ethyl hexanoate as-co-catalyst and with LiCl and polyglyame as additives employed in EP0648734 is not stable, not functional and can't afford a commercially viable process.
To solve the above problems, inventors developed a simpler and more functional catalyst system with benefits such as a long term stability, recoverability of the catalyst, and ease of product separation, while maintaining high catalyst activity and high reaction selectivity. Most importantly the newly developed Pd—Cu catalyst doesn't form inactive Pd—Cu precipitate during the reaction. The newly developed catalyst system is free of Cu salt precursors of higher carboxylic acids and hence there is no issue of co-production of undesired vinyl ester of higher carboxylic acid originating from copper salt. The new Pd—Cu catalyst system produces vinyl ester selectively, especially free of acid anhydrides of carboxylic acids during vinylation and which are commonly observed during Reppe vinylation catalysed by Zn-catalyst.